Automatic Repeat Request (ARQ) schemes are often used in packet communication systems to improve transmission reliability. Hybrid ARQ is a method that combines both FEC (forward error correction) and ARQ where previously unsuccessful transmissions are used in FEC decoding instead of being discarded. Hybrid ARQ enhances the effectiveness of FEC decoding and allows FEC blocks to be sent at high error rate operating points (S. B. Wicker, Error Control Systems for Digital Communication and Storage, Prentice-Hall, Inc., 1995).
One form of hybrid ARQ is “Chase” combining where the transmitter retransmits the same coded data packet (D. Chase, “Code Combing: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets,” IEEE Trans. on Commun., Vol. 33, pp. 593-607, May, 1985). The decoder at the receiver combines multiple copies of this transmitted packet in a certain manner. Another form is called incremental redundancy, where instead of sending simple repeats of the coded data packet, progressive parity packets are sent in each subsequent transmission of the packet. The decoder then combines packets with incremental information in an appropriate fashion and therefore decodes the packet at a lower code rate.
Hybrid ARQ normally involves the functionality at the physical layer and controls the FEC encoding and FEC decoding functions using an embedded physical layer fast feedback channel for control signaling. At times, the physical layer hybrid ARQ-FEC blocks may be retransmitted for the maximum number of times without success. Therefore, it alone cannot provide error free data delivery but permits operation at a lower signal-to-interference-plus-noise ratio (SINR).
Medium access control (MAC) ARQ is an error control feature which retransmits erroneous MAC packet data units (PDUs) in a flexible fashion to achieve error free data delivery. MAC-ARQ retransmissions may occur long after original transmission and the retransmission may be segmented and piggy backed on other MAC PDUs using the granularity of the defined ARQ block size.